A conventional apparatus 01 for treatment under a reduced pressure has a transfer robot 04 within a load-lock chamber 05, the robot 04 having a plurality of handling arms for conveying substrates to treatment chambers 02 and 03, as shown in FIGS. 6 and 7. In the conventional apparatus 01, for improving the productivity, the time required for the conveyance is shortened by increasing the operating speed of the apparatus.
However, the conventional apparatus uses the transfer robot for the conveyance of substrates under a reduced pressure. In this case, if the transfer robot operates in the atmosphere, the substrates are held by a vacuum suction, but under a reduced pressure, the substrates are merely placed on the handling arms, so it is impossible to effect a high-speed operation of the transfer robot. Thus, a limit has so far been encountered in shortening the conveyance time.
Further, in the case where the substrates are placed on cassette stages 07 and 08, which are held under atmospheric pressure, as shown in FIG. 6, it is necessary that the internal pressure of the load-lock chamber 05 with the transfer robot carried therein be returned to atmospheric pressure at every treatment of the two substrates. With the recent tendency to a larger substrate diameter, the load-lock chamber has become larger in volume, so that the time required for purge and evacuation is also a cause of a decrease in throughput. Even if the function of high-speed purge and high-speed evacuation is added to the conventional apparatus, the resulting high-speed current of air will cause displacement of the substrates on the handling arms or any other inconvenience caused by, for example, blowing-up of dust.
FIG. 7 shows an improvement over the above apparatus, in which the substrate cassettes 07 and 08 are accommodated within a load-lock chamber 09. Although the apparatus illustrated in FIG. 7 is advantageous in that the time required for purge and evacuation can be shortened, it involves many inconveniences in its application to an automatic substrate supply and recovery system developed along the recent tendency to factory automation. To meet this point, it is necessary to provide another system for automatic supply and recovery of the substrate cassettes with respect to the load-lock chamber. However, this results in an increase of cost. Further, due to an increase in volume of the cassette load-lock chamber with an increase in substrate diameter, the time required for purge and evacuation for each treatment lot has also come to be a cause of deteriorated productivity.
The present invention eliminates the above-mentioned drawbacks of the prior art and provides an apparatus for treatment under a reduced pressure capable of supplying substrates of a large diameter automatically and conveying them at a high speed and is capable of attaining a high throughput.